Normally NICs and switch/hub ports autoSense
speed and duplex settings. If the autoSense does not work properly, the
resulting mismatch will result in either extremely slow or no communications. To
correct this problem you can manually set the speed and duplex settings. Both
the NIC and the switch/hub port must have the same settings. The following
dialog shows the NIC properties where speed and duplex can be set.

NICs support different speed settings to be
compatible with old and new faster switch ports. Classical Ethernet is
half-duplex which means that the NIC cannot send and receive data at the same
time. Newer switches support full-duplex operation which supports sending and
receiving data at the same time.

ððóóHalf-Duplex hub
Full-Duplex switch

Frame Types

Ethernet sends octet bytes on a LAN local area
network in a grouping called an Ethernet frame. Some software supports
organizing the frame in different formats. This may lead to communications
problems if a client and server use a different frame format. Consistency is
important for successful communications. This is rarely a problem for TCP/IP
which normally used only the original DIX Ethernet frame.

Original DIX Dec/Intel/Xerox Ethernet frame

6 bytes

6

2

46-1500

4

Destination

Source

Type

Data

FCS

IEEE 802.3 Standard frame

6 bytes

6

2

46-1500

4

Destination

Source

Length

Data

FCS

IEEE 802.2 Standard frame

6 bytes

6

2

46-1500 of Data

4

Destination

Source

Length

DSAP SSAP

FCS

SNAP Subnetwork Access Protocol frame

6 bytes

6

2

46-1500 of Data

4

Destination

Source

Length

DSAP SSAP + EtherType

FCS

The FCS Frame Check Sequence is a checksum
calculated from data in the rest of the frame. The receiver verifies the
checksum and discards it if the checksum does not match the expected value.
Invalid checksums occur when data becomes corrupted by problems such as
electrical interference.

The Length field identifies the number of bytes
46-1500 in the Data field. The Type/EtherType field is a coded value that
indicates the Transport layer protocol within the Data field. Examples are IP or IPX.
The numeric Type values are greater than 1500. The value of the Type/Length
field identifies the frame format. Values greater than 1500 identify the IEEE
802.3 frame type and lower values identify the original DIX Ethernet frame type.

The IEEE Institute of Electrical and
Electronics Engineers sets international standards in data communications. The
DSAP and SSAP fields are the IEEE replacement for the EtherType field. The SNAP
frame has both the EtherType and the IEEE DSAP and SSAP fields so that it is
compatible with software that needs either standard.